Genes 22, 23, and 24 of bacteriophage T4 play roles in the shape control of the viral capsid. Mutations in these genes result in the production of capsids with abnormal, yet well-defined shapes. In gene 23 particularly a number of mutations altering capsid length are known and have been precisely mapped within that gene. This program has as its objective determination of the amino acid sequences of both normal and mutant gene23-coded proteins in the expectation that the comparison will provide some insight into the mechanism of capsid length control.